Energy-efficient head cell entry duct

ABSTRACT

A duct for conveying influent to a head cell having multiple, vertically aligned trays has an inlet end and an outlet end. The inlet end can be connected to an influent channel positioned at a level above the head cell apparatus. The outlet end is positioned downstream of the inlet end and can be connected to the head cell. The outlet end has multiple, discrete nozzles that are spaced apart in a vertical direction and correspond in number to the multiple trays. Influent that enters the inlet end of the duct travels downwardly and exits the duct through orifices in the nozzles, entering each of the respective trays.

BACKGROUND

The present invention relates to a head cell apparatus used inwastewater treatment, and in particular, to an energy-efficient duct andmethods used for conveying wastewater to be treated (i.e., influent) tothe head cell.

One phase of wastewater treatment is separating “grit,” which ishigh-density, inorganic, settleable particles, from the influent. Gritcauses wear to downstream treatment equipment and, if it accumulates,loss of performance.

One type of apparatus used for separating grit from influent is referredto as a head cell. Other approaches to removing grit have involved theuse of a horizontal mechanically rotating element (e.g., a paddle orpropeller) that circulates the influent within a surrounding cylindricaltank to separate the grit from the influent and cause it to gather in anaccumulating well. By way of contrast, head cells separate grit by acontinuous hydraulic action and do not require any mechanically-inducedmotion. Head cells are also self-cleaning.

Using a mechanically rotating element is disadvantageous because theperiodic nature of its rotation creates turbulence that tends tore-suspend finer grit. Also, larger objects that are typically found inan influent flow, such as rags, as one example, can accumulate and“bridge” operating areas in the well. In this case, such an apparatusmust then be drained, cleaned and/or repaired, which results indecreased treatment efficiency and increased operating costs.

The hydraulic separation action in a head cell occurs throughcontrolling the influent to flow at predetermined speeds and along apredetermined course, and does not require the use of chemicals. Theinfluent enters at the periphery or rim of a funnel-like conical surfacefrom a direction tangential to the rim, and then flows over and aroundthe downwardly sloping conical surface, at least partially circling acentrally located opening. The flow conditions are determined such thata dynamic boundary layer is developed at the conical surface.

As the influent flows around the downwardly sloping conical surface, thegrit is separated out onto the conical surface. At the same time, theremaining liquid, i.e., the effluent (which is relatively grit-freewastewater) is guided to flow out of the head cell through openingslocated at the outer periphery of the conical surface. In general, thiseffluent is channeled for further treatment downstream, e.g., as primarysludge.

At the same time, the separated grit moves downwardly along the slopingconical surface and through the opening for collection at a pointbeneath the opening. A head cell may have several individual conicalsurfaces or “trays” that are vertically aligned with each other suchthat grit draining through the central opening in an upper tray alsopasses through similar central openings in all lower trays. In a typicalhead cell having vertically aligned or “stacked” trays, a greaterworking surface area is provided relative to the head cell's footprintthan for comparably sized equipment having a single chamber with amechanically rotating element.

In some head cell installations, referred to herein as “upward feed headcells,” the influent is pumped vertically upward such that each of thestacked trays, in succession from a bottom tray to a top tray, receivesan amount of the influent through a peripheral inlet. The energyrequirement of this arrangement can be high due to the loss of influentvelocity head and the necessity of additional head to generate asuitable velocity in the peripheral inlet. In many installations,however, available head is limited, making this arrangement impractical.

Some wastewater treatment installations were originally implementedwithout grit removal equipment positioned upstream of the primary sludgetreatment equipment. Retrofitting such installations with grit removalequipment is desirable to eliminate or at least reduce the amount ofgrit in the primary sludge before it enters the primary treatmentequipment. Given the floor space constraints in existing installations,head cell equipment is often favored because it has a far superiorcapacity to remove grit (as great as 10 times more) per unit area of theequipment's footprint than the mechanically rotating element design.These retrofit installations, however, often have the same energylimitations that prevent use of an upward feed head cell.

SUMMARY

New methods and associated apparatus are provided for operating atreatment apparatus, e.g., a head cell, with improved performance andefficiency. Influent is fed downwardly to a head cell, which decreasesthe head cell operating energy (i.e., head) requirement and increasesefficiency. Because the new method requires less head, the head cell cannow be used in situations with low available head that were previouslyrestricted to mechanically rotating element equipment. Head cellsprovide superior performance over mechanically rotating elementequipment by removing smaller size grit and removing a higher percentageof grit in all other larger size ranges.

A new energy-efficient passageway member or duct is provided thatdirects influent downwardly to distribute it at multiple levels of ahead cell, from the top down. The new duct substantially minimizes headlosses and preserves the required flow conditions for proper operationof the apparatus. The duct reduces and may eliminate, in some cases, theexcess energy requirement for upward feed head cells in which theinfluent is pumped in an upward vertical direction.

According to some implementations, the duct directs or feeds a singleflow from a higher level downwardly and into multiple flows at lowerlevels substantially without any head loss. Influent may be directedfrom a higher level to each of the multiple trays of a head cell thatare positioned at lower levels without requiring an additional energyinput, e.g., to pump the influent or to power a grit-removingmechanically rotating element. Within a given energy limit, a head cellprovides superior performance because of its greater working surfacearea (i.e., the combined area of the multiple trays) per unit area offootprint than the working surface area of a mechanically rotatingelement system having a comparable footprint. The working surface areaof a mechanically rotating element system is limited to a portion of theinner planar surface of the cylindrical tank.

In commonly encountered retrofit situations, the lower performance ofmechanically rotating element equipment, sometime referred to as agravity grit chamber, would prevent removal of fine grit from thesystem's effluent before it enters other downstream processingequipment, e.g., primary treatment equipment. The remaining fine gritcauses undesirable wear. In the same situations, however, a downwardlyfed head cell removes substantially all of the fine grit.

The duct has an inlet end that is positioned higher than the outlet endsuch that influent will flow downwardly to the outlet end. The outlet isdivided into multiple, individual nozzles that are vertically separatedfrom each other. The nozzles terminate in openings or orifices throughwhich a portion of the influent is directed to each correspondingindividual tray of the head cell.

The outlet end of the duct is positioned to direct the influent flowingthrough the duct into the head cell from a direction generallytangential to the periphery of each tray.

The duct may have various sections along its length from the inlet end(i.e., where raw influent is received, e.g., from an open channel, tothe outlet end (i.e., where the duct joins the head cell). The duct mayhave a first section in which the velocity of the influent flow ischanged as necessary (e.g., by changing a cross-section of the firstsection), a second section that drops from a higher level to a lowerlevel and has a constant cross-section, and a third section in which theinfluent is distributed into multiple nozzles, which may be at differentlevels.

With the duct, influent is conveyed from a source at a higher level tomultiple lower levels, and the velocity of each multiple flow where itenters the treatment apparatus is maintained at a predetermined designvelocity. As necessary, the duct is configured to initially change theinfluent velocity to match the design velocity, and the influent is thenmaintained at this design velocity throughout the remaining downstreamlength of the duct. Advantageously, the influent is conveyed from thesource to the treatment apparatus substantially without any head loss.

Recitation of any aspect in this Summary of the Disclosure is notintended to imply that the aspect is an essential element. The Summaryis instead provided to facilitate understanding of the followingdetailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a perspective view of a conventional head cell installationhaving two head cell apparatus that each have a vertical influent pipethrough which influent is directed upwardly into multiple trays.

FIG. 1B is a side sectional view of the right side head cell apparatustaken along the line 1B—1B in FIG. 1A, showing the paths of theinfluent, separated grit and effluent through the multiple trays, withthe head cell apparatus shown submerged in a surrounding vessel.

FIG. 1C is a partial perspective sectional view taken from the region 1Cin FIG. 1B showing portions of the top three trays and the paths of theinfluent entering through influent openings into the trays.

FIG. 2 is a side view of a head cell apparatus with a newenergy-efficient duct for conveying influent to the head cell in adownward direction.

FIG. 3 is a top plan view of the head cell apparatus of FIG. 2.

FIG. 4A is a perspective view of the duct shown in FIG. 2.

FIG. 4B is a magnified side view of a portion of the duct of FIG. 4Ashowing the separation regions between adjacent nozzles.

FIG. 5 is a schematic top plan view of a head cell apparatus with awrapped-entry duct.

FIG. 6 is a schematic side view of the duct of FIG. 5.

FIG. 7 is a schematic top plan view of a head cell apparatus with astraight entry duct.

FIG. 8 is a schematic side view of the duct of FIG. 7.

FIG. 9 is a schematic top plan view of a head cell apparatus with anangled entry duct.

DETAILED DESCRIPTION

As described below, new methods and apparatus provide for operating atreatment apparatus with improved performance and efficiency. Inspecific implementations, the wastewater treatment apparatus is a headcell that removes grit from influent at an initial stage before theresulting effluent from the head cell is subjected to subsequenttreatment. Efficient grit removal requires achieving particular flowconditions in the head cell, which also implicates the design of theduct leading to the head cell.

Grit and Grit Removal

Removing grit from influent before subsequent treatment can helpalleviate two problems: (1) wear (especially of rotating parts), and (2)deposition and accumulation of grit that leads to loss of performance.As used herein, “grit” refers to inorganic, settleable solids that aredenser than water (i.e., having a specific gravity greater than 1.0).Grit includes both particles that travel along the bottoms of pipes,channels, ducts, etc., and particles that are suspended within theinfluent.

Unlike organic particles, which may be broken down in a downstreambiochemical process, inorganic particles tend to remain abrasive, thuscontributing to the wear problem unless removed. Settleable refers tothose particles that can be caused to settle for collection and removalunder conditions existing in a typical wastewater treatment plant.

Such inorganic settleable particles have a settling velocity that isapproximately equal to or greater than the settling velocity of a 50micron particle of silica sand (sometimes expressed as “50 micron SES”(Sand Equivalent Size)). The settling velocity for a 50 micron SESparticle in a quiescent tank is about 0.22 cm/sec. The settling velocityis a single parameter that allows the separability of particles havingdifferent specific gravities and sizes to be compared.

Conventional Head Cell Construction

A conventional head cell installation 10 is shown in FIG. 1A. Theconventional head cell installation 10 operates with an upward feed, andthus has an excess energy requirement.

The illustrated installation 10 has two head cell apparatus (or “headcells”) 11 a, 11 b that operate independently of each other, but areplaced adjacent each other as shown. The left side head cell 11 b hasthe same general construction and operation as the right side head cell11 a, which is described in detail below.

Referring to FIGS. 1A and 1B, the head cell 11 a has a number of trays12 (eight in this example) that are nested together. Depending upon theparticular application, a greater or lesser number of trays can be used.Each of the trays 12 has a circular rim 14 joined to a downwardlydirected conical section 16 having a central grit exit opening 20. Theconical section 16 has an inner sloping surface 17 a and an outersloping surface 17 b (visible for the bottom tray 12). The inner slopingsurface 17 a of each tray 12 is spaced apart from the outer slopingsurface 17 b of the immediately overlying tray 12, if any. The circularrim 14 of each tray 12 has a series of spaced effluent exit openings 18.

Referring to FIG. 1B, each tray 12 also has an annular-shaped baffle 15a extending inwardly and downwardly from the circular rim 14 with alarge, central baffle opening 15 b. The baffle 15 a is spaced above therespective conical surface 16, but below the level of the effluent exitopenings 18, thereby defining an influent circulation space 19 for therespective tray 12 between the inner sloping surface 17 a and an outersloping surface 15 c of the baffle 15 a. In the illustrated example, thebaffle 15 a extends downwardly at approximately the same angle as theconical section 16.

When multiple trays 12 are stacked together in alignment with each otheras shown, the baffle openings 15 b and the grit exit openings 20 of thetrays 12 are vertically aligned, as best seen in FIG. 1B.

Referring to FIGS. 1B and 1C, each of the trays 12 also has a peripheralinfluent entry opening 22 formed in the circular rim 14. As shown forthe top tray 12 in FIG. 1B, which has a portion of the baffle 15 aremoved for clarity, the influent opening 22 is approximatelyrectangular in cross-section. The influent opening 22 is positioned inthe rim 14 between the baffle 15 a and the conical section 16 (i.e.,within the influent circulation space 19 for the respective tray 12).Two such influent openings 22 are shown in FIG. 1C for the top two trays12.

Each tray 12 receives a portion of the influent flow travelling upwardthrough the vertical influent pipe 24 via the respective influent entryopening 22. As shown in FIG. 1A, the vertical influent pipe 24 has aninlet 25 a that receives influent to be treated, e.g., by a connectionto a pipe (not shown) located at a level below the lowest tray 12.

Head Cell Operation

In operation, the head cell apparatus 11 a is submerged within asurrounding chamber 21 a defined by a vessel wall 21 b (FIG. 1B) suchthat an upper surface 21 c of head cell effluent in the chamber 21 a isabove the level of the top tray 12.

Referring to FIG. 1A, the influent, which can be assumed to have auniform distribution of grit, is directed vertically upward through theinlet 25 a and the vertical influent pipe 24 (arrow A1 in FIG. 1A). Asthe influent flows upwardly, portions of it are successively distributedthrough the influent openings 22 to each of the respective trays 12.

The influent enters each of the trays 12 in a generally tangentialdirection (arrow A2 for the top tray 12), and begins to follows theinner periphery of the circular rim 14 within the influent circulationspace 19 beneath the baffle 15 a. This flow, which is referred to as aprimary flow, establishes a circular path (dashed arrows A3 and A4).

As the primary flow circulates around the conical surface 16, a dynamicboundary layer is established at its inner extent in a circular patternsurrounding the center of the conical section 16. This boundary layerinduces a secondary flow leading toward the grit exit opening 20.Heavier grit particles become subject to the secondary flow upon entryinto the tray 12, and are led by this flow through the grit exit opening20. Lighter grit particles begin to circulate around the influentcirculation space with the primary flow, but, due to gravity, eventuallysettle onto the conical surface and become subject to the secondaryflow.

As the influent flows in this manner, grit is separated out from theinfluent and onto the inner surface 17 a of the conical section 16 ofeach of the trays 12 (arrow B1). Referring to FIG. 1B, the separatedgrit travels downwardly through the grit exit opening 20 of therespective tray, and any underlying trays (arrows B2), and is collectedin a grit collection chamber 60 at the bottom of the head cell 11 a.

Meanwhile, the circulating influent, from which grit has been removed,is discharged from each of the trays 12 as effluent. The effluent isdischarged through the respective effluent exit openings 18. Aftercirculating partway around the tray 12, the effluent flows upward fromthe influent circulation space 19, through the respective baffle opening15 b and over the baffle 15 a, before exiting through the effluent exitopenings 18 (see arrows for the top tray in FIG. 1B).

This upward flow is the result of the primary flow in the circulardirection and the secondary flow in the downward direction. The influentflow entering the head cell 10 (through the influent openings 22 in thetrays 12) becomes the effluent flow following an upward path until it isdischarged from the head cell through the openings 18.

After the effluent is discharged, it is then directed elsewhere forsubsequent treatment or collection, as desired. In a typical head cell,the tray flow energy requirement or head loss due to its operation isabout 6 in.

Head Cell Operating Conditions

In the illustrated example, the energy required to elevate the influentupward from a level beneath the head cell 11 a and through the verticalinfluent pipe 24 to the head cell (i.e., the pump energy) may be 12 in.head (measured in water) or more. In certain cases, e.g., with openchannel flow, it may not be possible to provide sufficient energy toelevate the influent economically. Therefore, it would be desirable toreduce the energy required to introduce the influent into the head cell11 a, yet still maintain at least about 6 in. head required for headcell operation.

In operating head cells for a typical municipal wastewater facility,velocities for influent at points where the influent enters the trays 12(i.e., near the influent openings 22) may be from 1 ft./sec. to 10ft./sec. In a head cell in which the conical surfaces 16 are sloped atapproximately 45 degrees, premature settling may be observed at inletvelocities of less than about 5 ft./sec. It has been observed that inletvelocities of about 5 ft./sec. provide an adequate scour flow andsufficient settling performance in such a head cell. Inlet velocitiesabove 5 ft./sec. allow improved scour flow, but require greater than 6in. head.

New Head Cell Entry Duct

A new head cell entry duct can be used to channel influent from a levelabove the head cell (e.g., from an open channel) to each of theindividual trays of the head cell in a way that provides sufficientenergy and maintains suitable flow conditions for removing grit frominfluent. In contrast to the upward feed head cell, in which theinfluent flow entering the head cell is directed vertically upward, theinfluent flow with the new duct is fed downwardly.

FIGS. 2-4B show a head cell entry duct 26 according to a specificimplementation. In FIGS. 2 and 3, the head cell entry duct 26 is shownmounted to a head cell 11 a′, which is similar to the head cellapparatus 11 a, except the effluent exit openings 18′ (shown for thebottom two trays) are peripheral spaces between the trays 12 (instead ofthe openings 18 in the periphery of each tray 12). The trays 12 aremaintained in spaced alignment to each other by a surrounding framework13.

In FIGS. 2 and 3, the head cell 11 a′ is also shown in its operatingcondition within the chamber 21 b defined by the surrounding vessel ortank 21 a. The topmost tray 12 is submerged below the surface 21 c ofeffluent in the tank 21 a. As shown by the dashed lines within the duct26 in FIG. 2, a level 31 of the influent entering the head cell 11 a′from an open channel 30 is above the surface 21 c. As shown in FIG. 3,there is an exit duct 52 attached to the wall of the tank 21 a by whichthe effluent exits the head cell 11 a. A bottom surface 54 of the exitduct 52 is positioned at a level above the topmost tray 12.

The duct 26 has an inlet end 27 a and an outlet end 27 b. The inlet end27 a is adapted to connect to an influent supply, e.g., an open channel30, that is at a level above the head cell 11 a′ (i.e., the surface ofthe influent in the channel 30 is above the top tray 12). The outlet end27 b has multiple discrete nozzles 38 a that are arranged in thevertical direction to allow the duct 26 to be connected to therespective inlet opening 22 of each of the trays 12.

Influent is received into the duct 26 from the channel 30, and itsvelocity is changed to match a predetermined design velocity as it flowsalong the length of the duct 26. According to one aspect, the influentvelocity is increased to match the predetermined design velocity, andthis increased velocity is maintained substantially constant over theremaining length of the duct 26, including at downstream points wherethe flow is distributed into multiple flows and where the multiple flowsenter the head cell.

Referring to FIGS. 2-4A, in a flow direction from the inlet end 27 adownstream to the outlet end 27 b, the duct 26 in the illustratedimplementation has:

1. A first section (or adapter section) 28 extending downstream from theinlet end 27 a;

2. A second section (or drop section) 34 extending downstream from thefirst section; and

3. A third section (or distribution section) 36 extending downstreamfrom the second section and terminating in the discrete nozzles 38 a atthe outlet end 27 b.

In the first section 28, influent is received from the influent source.In the case of the channel 30, the influent velocity in the channel 30may be about 3 ft./sec. To meet the desired design velocity of about 5ft./sec. at the influent openings 22, the influent velocity must beincreased in the duct. (In other applications, the first section 28 maybe designed to decrease the influent velocity to match the designvelocity, or if the influent is already at the design velocity, tomaintain it.)

In the specific implementation shown, as interior passageway defined bythe first section 28 decreases in cross sectional area from the inletend 27 a to a first junction 32 with the second section 34. As shown inFIGS. 3 and 4A, the passageway of the first section 28 has anapproximately rectangular cross-section that decreases in area at asubstantially constant rate. Also, a bottom surface 28 a of the firstsection 28 slopes downwardly at a slight angle. In the specificimplementation, influent flowing through the duct 26 has a velocity ofabout 5 ft./sec. at the first junction 32.

In other implementations, the cross-section of the first section 28 maybe constant or may increase in the downstream direction. If the influentvelocity in the channel 30 is at the desired rate, the first section 28is formed with a constant cross-section. If the influent velocity in thechannel 30 is above the desired rate, the first section is formed with across-section that increases in the downstream direction.

The first section 28 can be partial or totally open (as shown) at itstop surface. As shown in FIGS. 3 and 4A, the first section 28 can haveside walls 29 that extend upward to a level above the channel 30 (to theupstream side) and above the second section 34 (to the downstream side).The walls 29 allow the first section 28 to accommodate surcharging(i.e., temporarily increased flow) from the channel 30.

Also, the open top surface of the first section 28 allows grease andother low density foreign matter in the influent, which tends to rise tothe top surface of the influent, to be removed before it enters the headcell apparatus.

The second section 34 slants downwardly from the first junction 32 withthe first section 28 such that a bottom surface of the second section 34terminates at a height approximately level with the bottom tray 12. Thesecond section 34 slants downwardly at a steep angle relative to thedownwardly sloping bottom surface 28 a of the first section 28.

The cross-section of a chamber defined by the second section 34 remainssubstantially constant in area, but changes in shape over the length ofthe second section 34. As illustrated in FIGS. 2 and 4, thecross-section of this chamber increases in vertical dimension anddecreases in horizontal dimension from the junction 32 downstream to asecond junction 35 where the second section joints the third section 36.The degree to which the cross-section of the chamber defined by thesecond section 34 changes in shape depends upon the steepness of thedrop from the level of the first section 28 to the level of the bottomtray 12.

At the second junction 35 where the second section 34 joins the thirdsection 36, the nozzles 38 a begin. The nozzles 38 a are substantiallyaligned in the vertical direction. Each nozzle 38 a defines a chamber 38b having an approximately rectangular cross-section that remainssubstantially constant over its length and terminates in an orifice oropening 38 c (FIG. 4B) at the outlet end 27 b. In the implementation ofFIGS. 2-4B, the chambers 38 b are formed to curve approximately 90° inthe horizontal plane. As indicated above, the orifices 38 c are formedto have approximately the same size and shape as the inlets 22.

Thus, the total cross-sectional area of all of the orifices 38 c isapproximately equal to the cross-sectional area of the chamber definedby the second section 34 at the first junction 32 and all downstreampoints along is length. As a result, the velocity of influent at each ofthe orifices 38 c/influent openings 22 is substantially equal to thevelocity of the total influent flow at the junction 32.

In the area of the second junction 35, the third section 36 may beformed to have curved separation regions 39 defined at interior areas ofthe duct 26 between adjacent nozzles 38 a. These separation regions 39may include projections 40 formed in the walls of the duct 26 betweenadjacent nozzles 38 a that extend outward into the flow in the upstreamdirection. These projections 40 smooth the distribution of the oncomingflow between the adjacent nozzles 38 a. Because the projections 40 arerounded, they also inhibit objects in the influent from becoming lodgedthere in the spaces between the nozzles 38 a under the force of theinfluent flow.

As shown in FIG. 4B, each projection 40 in a specific implementation hascurved upper and lower sides 50 a, 50 b, respectively, with the upperside 50 a being more curved than the lower side 50 b. Specifically, theupper side 50 a may have a curve with a radius of approximately 1 in.relative to the distance separating the adjacent nozzles 38 a.Similarly, the lower side 50 b may have a curve with a balancing,greater radius relative to the distance separating the nozzles 38 a.These radii depend upon the nozzle-to-nozzle spacing and nozzle depth ofany specific application.

In the side view of the head cell 11 a′ of FIG. 2, there is a gussetplate 59 formed at the second junction 35 and an adjacent outer skinthat conceal the separation regions 39 from view.

Referring to FIG. 2, the topmost tray 12 of a specific implementation issubmerged by a depth d below the surface 21 c by about 3 ft. A level ofinfluent 23 within the duct 26 near the inlet end 27 a is a distance eabove the surface 21 c. In this implementation, the distance e is about6 in.

In an exemplary application, the diameter of each tray 12 isapproximately 9 ft., the diameter of each baffle opening 15 b isapproximately 7 ft., the diameter of each grit exit opening 20 isapproximately 2 ft., and each baffle 15 a and conical section 16 aresloped at an angle of about 45 degrees. Adjacent inlet openings 22 arespaced approximately uniformly in a vertical direction by about 1 ft. Inthe same example, the inlet openings 22/orifices 38 c each have across-sectional area of about 12 square in.

The total drop in height from the inlet end 27 a of the duct to theoutlet end 27 b can be set according to the particular requirements of agiven installation (e.g., the height difference between the channel 30and the head cell 11 a, the available space in which to fit the duct 26,etc.). Similarly, the drops in height from the inlet end 27 a to thefirst junction 32 and from the first junction 32 to the second junction35 can also be set as required. As one example, referring to FIG. 2,these drops in height are about 6 in. from the inlet end 27 a to thefirst junction 32 and about 3.5 ft. from the first junction 32 to thesecond junction 35. Of course, the specific drops in height will dependupon the number of nozzles and nozzle-to-nozzle spacing of theparticular application.

New Head Cell Duct/Head Cell Performance

With a downward feed head cell, e.g., using the duct 26, performance issignificantly improved. As described, in a downward feed head cell witha substantially loss-free duct, only as much as 6 in. head may berequired for operation. This 6 in. head requirement is substantially dueto head losses inherent in the operation of the head cell itself, andnot from any losses due to conveying the influent to the head cell.

The head cell has a substantially greater working surface (i.e., thecombined area of all of the trays) than the working surface of aconventional grit removal apparatus having a mechanically rotatingelement within a cylindrical chamber, which is just a portion of theinner surface of the chamber. Also, as opposed to continuous hydraulicaction of the head cell, the cyclical action of the mechanicallyrotating element causes turbulence that re-suspends grit particles, thuslowering the efficiency of the conventional apparatus. For a givenfootprint, a head cell may provide ten times the grit removing capacityas a conventional apparatus. Stated differently, a head cell of a givenfoot print may be capable of or removing smaller grit particles (e.g.,50 micron SES), which are more difficult to remove, whereas a comparableconventional apparatus may only be capable of removing larger particles(e.g., 250-300 micron SES).

In addition, the downward feed head cell is also more efficient atremoving grit for systems that experience wide fluctuations between PeakWet Weather Flow (PWWF) and Average Dry Weather Flow (ADWF).

Miscellaneous Considerations

In the illustrated implementations, each section of the duct 26 has arectangular cross-section, and each section is enclosed by a top surfaceor panel. Depending upon the particular requirements, some sections ofthe duct 26 may be configured to have other cross-sectional shapes,e.g., a circular or other form of curved cross-section.

As shown schematically in FIGS. 2-9, the duct 26 can be configured forwrapped entry (FIGS. 2-6), straight entry (FIGS. 7 and 8) or angledentry (FIG. 9). With a wrapped entry configuration as shown in FIG. 4A,the walls of the nozzles 38 a must be curved such that the orifices 38 cintersect the respective influent entry openings 22. As illustrated, thenozzles 38 a follow a curve of approximately 90 degrees. As would beknown to one of ordinary skill in the art, the particular configurationfor the duct 26 would depend upon space limitations and relativegeometrical considerations between the influent source and theassociated treatment apparatus.

The duct 26 is designed for use with head cells of different sizes.Specifically, a particular duct configured for one head cell can be usedwith another head cell having the same number of trays, same trayspacing and same sized inlet openings. As shown for the head cell 11 a′in FIG. 2, there is an adapter section 56 for each tray 12 to which therespective nozzle 38 a is joined. The adapter section 56 defines arectangular opening having a horizontal dimension approximately half ofthe diameter of the tray 12 and a height approximately equal to theheight of the associated inlet opening 22. Adapter plates 58 a, 58 b areattached to the adapter section 56 at either side of the inlet opening22 to cover and seal the remaining exposed space of the rectangularopening, thereby completing the junction between the duct 26 and thehead cell 11 a. For a different head cell as described above (i.e. ahead cell having a different diameter), only the adapter plates 58 a, 58b need to be changed.

In a specific implementation, the duct is formed of sheet metal.Alternatively, various other materials may be used, e.g., plastic orother suitable material. The trays 12 may be constructed of ahydrophobic material to resist grease accumulation. In a specificimplementation the trays 12 are constructed of polyethylene.

Having illustrated and described the principles of our invention withreference to several preferred embodiments, it should be apparent tothose of ordinary skill in the art that the invention may be modified inarrangement and detail without departing from such principles.

What is claimed is:
 1. In a head cell apparatus having plural verticallyaligned trays that each receive and treat wastewater, a duct forconveying influent to be treated to the apparatus, the duct comprising:a passageway that has an open top surface along at least a portion ofits length and an inlet end connectible to an open influent channelpositioned at a level above the head cell apparatus to receive an openchannel flow of influent and convey the open channel flow downstream;and an outlet end positioned downstream of the inlet end and connectableto the head cell apparatus, the outlet end comprising a plurality ofdiscrete nozzles corresponding in number to the plurality of trays, thenozzles being spaced in a vertical direction and each having at leastone orifice, the orifices being positionable relative to the trays suchthat influent entering the inlet end of the duct travels downwardly andexits the outlet end through the orifices into the respective trays. 2.The duct of claim 1, wherein the nozzles define respective chambers eachhaving a substantially rectangular cross-section.
 3. The duct of claim1, wherein at least one nozzle is separated from an adjacent nozzle by aseparation region.
 4. The duct of claim 3, wherein the at least oneseparation region is defined by a rounded projection extending from twoadjacent nozzles and directed in a generally upstream direction.
 5. Theduct of claim 4, wherein the projection has an upper side and a lowerside, and wherein the upper side is more rounded than the lower side. 6.The duct of claim 1, further comprising an adapter section extendingdownstream from the inlet end, the adapter section defining a passagewayhaving a cross-sectional area that changes over a length of the adaptersection.
 7. The duct of claim 6, wherein the cross-sectional area of thepassageway of the adapter section generally decreases in the downstreamdirection.
 8. The duct of claim 6, wherein an interior of the adaptersection has a sloped bottom surface.
 9. The duct of claim 6, wherein aninterior of the adapter section has a bottom surface that slopesdownwardly in the downstream direction.
 10. The duct of claim 6, whereinat least part of the adapter section has an open top.
 11. The duct ofclaim 1, further comprising a drop section that defines a chamber havinga substantially constant cross-sectional area along its length.
 12. Theduct of claim 6, wherein a cross-section of the passageway changes overa length of the adapter section in the downstream direction.
 13. Theduct of claim 12, wherein the cross-section of the passageway decreasesin vertical dimension and increases in horizontal dimension in thedownstream direction.
 14. The duct of claim 1, further comprising adistribution section that terminates at the orifices of the nozzles atthe outlet end of the duct, the distribution section having an upstreamportion defining a single chamber from which influent flowingtherethrough is distributed among the multiple nozzles.
 15. The duct ofclaim 14, wherein the distribution section defines chambers that extendalong a curved path in a horizontal plane.
 16. The duct of claim 14,wherein the distribution section defines chambers that extend along anangled path in a horizontal plane.
 17. The duct of claim 14, wherein thedistribution section defines chambers that extend along a substantiallystraight path in the horizontal plane.
 18. A head cell entry duct forconveying influent to be treated to a head cell having a plurality ofvertically spaced trays, the duct comprising: an inlet end connectibleto an open channel influent source positioned at a level above the headcell apparatus; a first section extending from the inlet end downstreamin a flow direction to a first junction, the first section defining apassageway having a cross-sectional area that decreases in the flowdirection, the passageway having a top surface that is open over atleast a portion of a distance between the inlet end and the firstjunction; a second section adjacent and downstream of the first section,the second section defining an interior chamber and terminating at alevel below the first section; and a third section adjacent anddownstream of the second section, the third section having a respectiveplurality of nozzles that each define a chamber terminating at anorifice at the outlet end of the duct, the nozzles being connectible tothe plurality of trays of the head cell apparatus.
 19. A downward feedhead cell assembly for removing grit from wastewater, comprising: aplurality of vertically-spaced trays, each of the trays having an upperperiphery, a downwardly directed conical portion with a terminal enddefining an exit opening, and an inlet opening formed in a side of thetray adjacent the upper periphery; a passageway through which influentis conveyed from a source at a level above a topmost one of the trays toeach of the trays, the passageway having an open top surface over atleast a portion of its length, the passageway having au inlet end thatreceives influent from the source and an outlet end lower than the inletend, the outlet end having a corresponding plurality of nozzles, each ofthe nozzles being attached to a respective tray at the inlet opening,whereby influent enters the passageway as an open channel flow and isdistributed into multiple flows through the nozzles and into the trays,the influent entering each tray through the inlet opening andestablishing a flow that follows the conical surface, the grit in theinfluent settling toward the exit opening in the tray while thegrit-reduced influent is discharged as effluent through the spacebetween the tray and any overlying tray.
 20. The head cell assembly ofclaim 19, wherein the passageway is substantially head loss free. 21.The head cell assembly of claim 19, wherein the passageway has a sectionwith a decreasing cross-sectional area that increases a velocity ofinfluent flowing through the first section to a predetermined designvelocity.
 22. The head cell assembly of claim 21, wherein thepredetermined design velocity is about 5 ft./sec.
 23. A head cell entryduct, comprising: a first section adapted to receive an entering openchannel flow of influent at a first elevation; a second section that ispositioned downstream of and slopes downwardly from the first section,the second section having a second section cross sectional area in aplane normal to a direction of flow in the second section that remainssubstantially constant over a length of the second section; and a thirdsection positioned downstream of the second section and terminating inmultiple nozzles, the nozzles having a closed cross-section and beingarranged at different elevations lower than the first elevation, thenozzles having a total cross sectional area substantially equal to thesecond section cross sectional area, the third section being shaped toreceive the flow from the second section and to distribute the flowthrough the multiple nozzles as full-pipe flow.